The present invention relates to a seat belt retractor comprising a retractor frame, a belt spool rotatably mounted in the frame, an electric drive motor and a reduction gear coupling the electric motor to the belt spool.
Conventional seat belt retractors have a winding spring permanently engaged between the frame and the belt spool to bias the belt spool in the winding direction. The winding spring must be dimensioned to overcome frictional resistance from various sources such as deflection rings and passengers cloths over which the belt webbing slides, thus ensuring the belt being substantially free of slack. On the other hand, belt tension is detrimental to comfort and, in fact, is one of the reasons for not fastening a seat belt. Sophisticated mechanisms have been developed to reduce belt tension over a limited range of belt length withdrawn from the spool to enhance comfort.
Another approach is to replace the winding spring with an electric drive motor. The electric drive motor can be easily controlled to develop appropriate belt tension in all circumstances, including pre-crash tensioning of the seat belt. In the event of an electric power failure, however, no winding function is available, and the seat belts cannot be used, nor can they be stowed away by winding on their belt spools.
The present invention provides a seat belt retractor with an electric drive motor wherein a winding function is available even in case of an electric power failure. According to the invention, the belt retractor comprises a retractor frame, a belt spool rotatably mounted in the frame, an electric drive motor and a reduction belt gear with a toothed belt connecting the electric motor permanently with the belt spool. A winding spring is functionally arranged between the frame and the belt spool, permanently biasing the belt spool with a winding moment. The electric motor is controlled to either counteract or assist the winding spring. In case of an electric power failure, the winding spring must drive the belt spool and simultaneously entrain the electric motor through the reduction gear that now acts as a step-up gear. As a first requirement, the reduction gear must be reversible, i.e. it must transmit rotation in both directions. A second requirement is that the reduction gear should have a high efficiency thereby limiting the necessary spring force. A belt gear inherently satisfies both requirements.
In the preferred embodiment of the invention, a length of belt webbing withdrawn from the belt spool is detected. The length of belt webbing withdrawn is compared to predetermined threshold values to discriminate between a belt wearing condition and a belt non-wearing condition. The electric motor is driven in the wearing condition with current of a first polarity and adjusted to counteract the winding spring for appropriate belt wearing comfort, and is driven in a non-wearing condition with current of a second, opposite polarity adjusted for full retraction of belt webbing on the belt spool. Thus, belt tension can be adjusted for an optimum comfort after an initial tensioning to remove excessive belt slack, and increased in the non-wearing condition to safely retract, in combination with the winding spring, the belt webbing on the belt spool. The winding spring is dimensioned to overcome frictional resistance and mass inertia in the entire belt system, including the reduction gear and the electric motor.